Having drawn up some notes for a French translation of my work 'On
the various contrivances by which British and Foreign Orchids are
Fertilized by Insects' (1862), it has appeared to me that these notes
would be worth publishing in English.1 I have thus been able to bring up
the literature of the subject to the present day, by giving references
to, together with very brief abstracts of, all the papers published
since my work appeared. These papers contain, on the one hand,
corrections of some serious errors into which I had fallen, and, on the
other hand, confirmations of many of my statements. I have also been
able to add, from my own observations and those of others, a few new
facts of interest. A heading is given to each note, which will show the
nature of the correction or addition, without any reference to my book;
but I have added in a parenthesis the page to which the note ought to
be appended.

Gentlemen,

Your obedient Servant,

CHARLES DARWIN.

Down, Beckenham, Kent.
July 23, 1869.

Orchis or Anacamptis pyramidales (p. 20).—The
late Prof. Treviranus2 has confirmed (Botanische Zeitung, 1863, p. 241) my observations on this remarkable species; but he differs from me in
one or two minor points.

On the kinds of Insects which habitually visit and fertilize
some of the common British species of Orchis (p. 35).—I believe

that it may be safely predicated that orchids with very long
nectaries, such as the Anacamptis, Gymnadenia, and Platanthera,
are habitually fertilized by Lepidoptera, whilst those with only
moderately long nectaries are fertilized by bees and Diptera—in short,
that the length of the nectary is correlated with that of the proboscis
of the insect which visits the plant. I have now seen Orchis morio
fertilized by various kinds of bees, namely:—by the hive-bee (Apis
mellifica), to some of which from ten to sixteen pollen-masses
were attached; by Bombus muscorum, with several pollen-masses
attached to the bare surface close above the mandibles; by Eucera
longicornis,with eleven pollen-masses attached to its
head; and by Osmia rufa. These bees, and the other
Hymenoptera mentioned throughout these notes, have been named for me by
our highest authority, Mr. Frederick Smith, of the British Museum. The
Diptera have been named by Mr. F. Walker, of the same establishment. In
Northern Germany, Dr. H. Müller1 of Lippstadt found pollen-masses of Orchis
morio attached to Bombus silvarum, lapidarius,
confusus,
and pratorum. The same excellent observer found the
pollen-masses of Orchis latifolia attached to a Bombus;
but this orchis is also frequented by Diptera. A friend watched for me Orchis
mascula, and saw several flowers visited by a Bombus,
apparently B. muscorum; but it is surprising how
seldom any insect can be seen visiting this common species. With
respect to Orchis maculata, my son, Mr. George Darwin, has
clearly made out the manner of its fertilization. He saw many specimens
of a fly (Empis livida) inserting their proboscides into the
nectary; and subsequently I saw the same occurrence. He brought home
six specimens of this Empis, with pollinia attached to their
spherical eyes, on a level with the bases of the antennæ. The pollinia
had undergone the movement of depression, and stood a little above and
parallel to the proboscis: hence they were in a position excellently
adapted to strike the stigma. Six pollinia were thus attached to one
specimen, and three to another. My son also saw another and smaller
species (Empis pennipes) inserting its proboscis into the
nectary; but this species did not act so well or so regularly as the
other in fertilizing the flowers. One specimen of this latter Empis
had five pollinia, and a second had three pollinia, attached to
the dorsal surface of the convex thorax.

On nectar being secreted and contained between the outer and
inner membranes of the nectary in several species of Orchis (p.
51).—I have repeated my observations on the nectaries of some of our
common species, and especially on those of Orchis morio,
at the time when various bees were continually visiting

the flowers; but I could never see the minutest drop of nectar
within the nectary. Each bee remained a considerable time with its
proboscis in constant movement whilst inserted into the nectary. I
observed the same fact with Empis in the case of Orchis
maculata; and in this orchis I could occasionally detect minute
brown specks, where punctures had been made. Hence the view suggested
by me that insects puncture the inner lining of the nectary and suck
the fluid contained between the two coats may be safely accepted. I
have said in my work that this hypothesis was a bold one, as no
instance was known of Lepidoptera penetrating with their delicate
proboscides any membrane; but I now hear from Mr. R. Trimen1 that at the
Cape of Good Hope moths and butterflies do much injury to peaches and
plums by penetrating the skin, in parts which have not been in the
least broken.

Since the appearance of my work, the following observations have
been published on other species of Orchis and on certain
allied forms (p.
53).—Mr. J. Traherne Moggridge2 has given (Journ. Linn. Soc. vol. viii.
Botany, 1865, p. 256) a very interesting account of the structure and
manner of fertilization of Orchis or Aceras
longibracteata. Both pollinia, as in Anacamptis pyramidalis,
are attached to the same viscid disk; but, differently from those in
that species, after being removed from the anther-cases, they first
converge and then undergo the movement of depression. But the most
interesting peculiarity in this species is that insects suck nectar out
of minute open cells in the honeycombed surface of the labellum. Mr.
Moggridge saw this plant fertilized by a large bee, the Xylocopa
violacea. He adds some observations on Orchis hircina,
and describes the structure and manner of fertilization of Serapias
cordigera by another bee, viz. the Ceratina albilabris. In
this Serapias both pollinia are attached to the same viscid
disk; when first withdrawn, they are bent backwards, but soon
afterwards move forwards and downwards in the usual manner. As the
stigmatic cavity is narrow, the pollinia are guided into it by two
guiding plates.

Mr. Moggridge sent me from Northern Italy living plants of Orchis
or Neotinea intacta, together with excellent drawings
and a full account of the structure of the flower. He informed me that
this species is remarkable for producing seed without the aid of
insects; and I ascertained that when insects were carefully excluded,
almost all the flowers produced capsules. Their fertilization follows
from the pollen being extremely incoherent, and spontaneously falling
on the stigma. Nevertheless a short nectary is present, the pollinia
possess

small viscid disks, and all the parts are so arranged that, if
insects were to visit the flowers, the pollen-masses would probably be
removed and then carried to another flower, but not so effectually as
with most other orchids. We shall hereafter find a few other cases of
orchids which have structural peculiarities adapted both for
self-fertilization and for crossing. I may here also refer to a paper
by Mr. R. Trimen (Journ. Linn. Soc. vol. vii. Botany, 1863, p. 144)1 on
the beautiful Disa grandiflora of the Cape of Good Hope.
This orchid presents several remarkable characteristics, one of these
being that the pollinia do not spontaneously undergo any movement of
depression, the weight of the pollen-masses sufficing to bend the
caudicle into the proper curvature for the act of fertilization.
Another peculiarity is that the posterior sepal secretes nectar, and is
developed into a spur-like nectary. Mr. Trimen informs me that he has
seen a Dipterous insect, allied to Bombylius, frequenting the
flowers. I may add that Mr. Trimen has sent me descriptions and
specimens of various other South-African orchids, which confirm the
general conclusions at which I have arrived in my work.

On the movement of the pollinia of Ophrys muscifera (p.
56).—Mr. T. H. Farrer,2 who has lately been attending to the
fertilization of various plants, has convinced me that I have erred,
and that the pollinia of this Ophrys do undergo a movement
of depression. Hence my remarks on the correlation of the various parts
of the flower are to a certain extent invalidated; but there can be no
doubt that the naturally bent caudicle plays an important part in
placing the pollen-mass in a proper position for striking the stigma. I
have continued occasionally to watch the flowers of this species, but
have never succeeded in seeing insects visit them; but I have been led
to suspect that they puncture or gnaw the small lustrous prominences
beneath the viscid disks, which, I may add, are likewise present in
several allied species. I have observed very minute punctures on these
prominences, but I could not decide whether these had been made by
insects or whether superficial cells had spontaneously burst.

Ophrys aranifera (p. 63).—F. Delpino3 states (Fecondazione
nelle Piante &c., Firenze, 1867, p. 19) that he has examined in
Italy thousands of specimens of this Ophrys, and that it
seldom produces capsules. It does not secrete any nectar. Although he
never saw an insect on the flowers (excepting once a green locust),
nevertheless they are fertilized by insects; for he found pollen on the
stigmas of some flowers, which had their own pollinia still within the
anther-cases. The pollinia never

spontaneously fall out. He appears to think that I infer that this Ophrys
fertilizes itself, which is an error.

Ophrys apifera (p. 71).—Prof. Treviranus at first doubted
(Botanische Zeitung, 1862, p. 11)1 the accuracy of my account of this Ophrys,
and of the differences between it and O. arachnites;but
he has subsequently (Bot. Zeit. 1863, p. 241) fully confirmed all that
I have stated.

Ophrys arachnites (p. 72).—I have now examined several
additional living specimens of this Ophrys, and can confirm
my statement that the pollinia do not fall out of the anther-cases,
even when the spikes are strongly shaken; nor do they fall out when the
spikes are kept standing in water for a week. Mr. J. Moggridge has
made (Journ. Linn. Soc., Bot. vol. viii. 1865, p. 258) a remarkable
observation on O. scolopax, which is closely allied to O.
arachnites,—namely, that at Mentone it never exhibits any tendency
to self-fertilization, whilst at Cannes all the flowers fertilize
themselves, owing to a slight modification in the curvature of the
anther, which causes the pollinia to fall out. This botanist has given,
in his 'Flora of Mentone,'2 a full description, with excellent figures,
of O. scolopax, arachnites, aranifera, and apifera;
and he believes, from the number of intermediate forms, that they must
all be ranked as varieties of a single species, and that their
differences are intimately connected with their period of flowering. It
does not appear that these forms in England, judging from their
distribution, are liable to pass into each other, within any moderate
or observable period of time.

On the fertilization of Herminium monorchis (p. 74).—My
son, Mr. George Darwin, has fully observed the manner of fertilization
of this minute and rare orchis. It differs from that of any other genus
known to me. He saw the flowers entered by various minute insects, and
brought home no less than twenty-seven specimens with pollinia
(generally with only one, but sometimes with two) attached to them.
These insects consisted of minute Hymenoptera (of which Tetrastichus
diaphantus was the commonest), of Diptera and Coleoptera, the
latter being Malthodes brevicollis. The one indispensable
point appears to be that the insect should be of very minute size, the
largest being only the 1/20 of an inch in length. It is an
extraordinary fact that in all the specimens the pollinia were attached
to the same peculiar spot, namely, to the outer side of one of the two
front legs, to the projection formed by the articulation of the femur
with the coxa. In one instance alone a pollinium was attached to the
outside of the femur a little beneath the articulation. The cause of
this peculiar manner of attachment is sufficiently clear: the middle
part

of the labellum stands so close to the anther and stigma, that
insects always enter the flower at one corner, between the margin of
the labellum and one of the upper petals; they also almost always crawl
in with their backs turned directly or obliquely towards the labellum.
My son saw several which had begun to crawl into the flower in a
different position; but they came out and changed their position. Thus,
standing in either corner of the flower, with their backs turned
towards the labellum, they inserted their heads and fore legs into the
short nectary, which is seated between the two widely separated viscid
disks. I ascertained that they stand in this position by finding three
dead insects, which had been permanently glued to the disks. Whilst
sucking the nectar, which occupies about two or three minutes, the
projecting joint of the femur stands under the large helmet-like viscid
disk on either side; and when the insect retreats, the disk exactly
fits on, and is glued to, the prominent joint. The movement of
depression in the caudicle then takes place, and the mass of
pollen-grains projects just beyond the tibia; so that the insect, when
entering another flower, can hardly fail to fertilize the stigma, which
is situated directly beneath the disk on either side. I know of hardly
any other case in which the whole structure of the flower is more
beautifully correlated than in the Herminium for a most
peculiar manner of fertilization.

On the movement of the pollinia in Peristylus viridis (p.
76).—Mr. T. H. Farrer informs me that the pollinia certainly undergo a
movement of depression, but that this does not take place until twenty
or thirty minutes have elapsed after their removal from the
anther-cases. This length of time probably accounts for my oversight.
He asserts that, after the movement of depression, the pollinia become
much better adapted to strike the stigmatic surface. He suggests that
insects may take a long time to lick up the nectar from the two naked
spots on the labellum, and through the narrow slit-like opening into
the nectary—and that during this time the pollinium becomes firmly
attached, by the slow hardening of the viscid matter, to the insect's
body, so as to be subsequently ready to fertilize another flower when
visited by the same insect.

On the Lepidoptera which fertilize the Gymnadenia
conopsea, and on the divergence of the pollinia (p. 82).—Mr.
George Darwin went at night to a bank where this species grows
plentifully, and soon caught Plusia chrysitis with six
pollinia, P. gamma with three, Anaitis plagiata with
five, and Triphæna pronuba with seven
pollinia attached to
their proboscides.

I may add that he caught the first-named moth, bearing the pollinia
of this orchis, in my flower-garden, although more than a quarter of a
mile distant from any spot where the plant grows. I state in my work
that I do not understand the cause of the divergence of the pollinia
so that they are enabled to strike the lateral stigmatic surfaces; but
the explanation is simple. The upper margin of the nectary is arched,
being formed on one side by the disk of one pollinium, and on the other
side by the other disk. Now if a moth inserts its proboscis obliquely,
and there are no guiding-ridges by which, as in Anacamptis
pyramidalis, a moth is compelled to insert its proboscis directly
in front, or if a bristle be inserted obliquely, one pollinium alone is
removed. In this case the pollinium becomes attached a little on one
side of the bristle or proboscis; and its extremity, after the vertical
movement of depression, occupies a proper position for striking the
lateral stigma on the same side.

On the Gymnadenia tridentata of North America (p.
83).— Prof. Asa Gray has published (American Journal of Science, vol.
xxxiv. 1862, p. 426, and footnote p. 260; and vol. xxxvi. 1863, p. 293)1some interesting notes on the Gymnadenia tridentata.The
anther opens in the bud, and some of the pollen invariably falls on the
naked cellular tip of the rostellum; and this part, strange to say, is
penetrated by the pollen-tubes, so that the flowers are
self-fertilized. Nevertheless "all the arrangements for the removal of
the pollinia by insects (including the movement of depression) are as
perfect as in the species which depend upon insect aid." Hence there
can be little doubt that this species is occasionally crossed.

Habenaria or Platanthera bifolia (p.
88).—According to Dr. H. Müller, of Lippstadt, Pl. bifolia of
English authors is the Pl. solstitialis of Boenninghausen;
and he fully agrees with me that it must be ranked as specifically
distinct from Pl. chlorantha.Dr. Müller states
that this
latter species is connected by a series of gradations with another form
which in Germany is called Pl. bifolia.He gives a
very full
and valuable account of the variability of these species of Platanthera
and of their structure in relation to their manner of
fertilization. (See Verhandl. d. Nat. Verein. Jahrg. xxv. III. Folge,
v. Bd. pp. 36-38.)

American species of Platanthera (p. 91).—Prof. Asa Gray
has described (American Journal of Science, vol. xxxiv. 1862, pp. 143,
259, & 424, and vol. xxxvi. 1863, p. 292) the structure of ten
American species of Platanthera.Most of these
resemble in
their manner of fertilization the two British species described by me;
but some of them, in which the viscid

disks do not stand far apart, have curious contrivances, such as a
channelled labellum, lateral shields, &c., compelling moths to
insert their proboscides directly in front. Pl. Hookeri, on
the other hand (ibid. vol. xxxiv. 1862, p. 143), differs in a
very interesting manner: the two viscid disks stand widely separated
from each other; consequently a moth, unless of gigantic size, would be
able to suck the copious nectar without touching either disk; but this
risk is avoided in the following manner:—The central line of the stigma
is prominent, and the labellum, instead of hanging down, as in most of
the other species, is curved upwards, so that the front of the flower
is made somewhat tubular and is divided into two halves. Thus a moth is
compelled to go to one or the other side, and its face will almost
certainly be brought into contact with one of the disks. The drum of
the pollinium, when removed, contracts in the same manner as I have
described under Pl. chlorantha. Prof. Gray has seen a
butterfly from Canada with the pollinia of this species attached to
each eye. In the case of Platanthera flava (American Journal
of Science, vol. xxxvi. 1863, p. 292), moths are compelled in a
different manner to enter the nectary on one side. A narrow but strong
protuberance, rising from the base of the labellum, projects upwards
and backwards, so as almost to touch the column; thus the moth, being
forced to go to either side, is almost sure to withdraw one of the
viscid disks. In the allied and wonderful Bonatea speciosa of
the Cape of Good Hope there is a similar contrivance for the same
purpose.

Platanthera hyperborea and dilatata have been
regarded by some botanists as varieties of the same species; and Prof.
Asa Gray says (Amer. Journ. of Science, vol. xxxiv. 1862, pp. 259 &
425) that he has often been tempted to come to the same conclusion; but
now, on closer examination, he finds, besides other characters, a
remarkable physiological difference, namely, that Pl. dilatata,
like its congeners, requires insect aid and cannot fertilize itself;
whilst in Pl. hyperborea the pollen-masses commonly fall out
of the anther-cells whilst the flower is very young or in bud, and thus
the stigma is self-fertilized. Nevertheless the various structures
adapted for crossing are still present.

Fertilization of Epipactis palustris (p. 102).—My son, Mr.
W. E. Darwin, has carefully observed for me this plant in the Isle of
Wight. Hive-bees seem to be the chief agents in fertilization; for he
saw about a score of flowers visited by these insects, many of which
had pollen-masses attached to their foreheads, just above the
mandibles. I had supposed that insects crawled into the flowers; but
hive-bees are too large to

do this; they always clung, whilst sucking the nectar, to the distal
and hinged half of the labellum, which was thus pressed downwards.
Owing to this part being elastic and tending to spring up, the bees, as
they left the flowers, seemed to fly rather upwards; and this would
favour, in the manner explained by me, the complete withdrawal of the
pollen-masses, quite as well as an insect crawling out of the flower in
an upward direction. Perhaps, however, this upward movement may not be
so necessary as I had supposed; for, judging from the point at which
the pollen-masses were attached to the bees, the back part of the head
would press against, and thus lift up, the blunt, solid, upper end of
the anther, thus freeing the pollen-masses.

Various other insects besides hive-bees visit this Epipactis. My
son saw several large flies (Sarcophaga carnosa) haunting the
flowers; but they did not enter in so neat and regular a manner as the
hive-bees; nevertheless two had pollen-masses attached to their
foreheads. Several smaller flies (Cœlopa frigida)
were also
seen entering and leaving the flowers, with pollen-masses adhering
rather irregularly to the dorsal surface of the thorax. Three or four
distinct kinds of Hymenoptera (one of small size being Crabro
brevis) likewise visited the flowers; and three of these
Hymenoptera had pollen-masses attached to their backs. Other still more
minute Diptera, Coleoptera, and ants were seen sucking the nectar; but
these insects appeared to be too small to transport the pollen-masses.
It is remarkable that some of the foregoing insects should visit these
flowers; for Mr. F. Walker informs me that the Sarcophaga frequents
decaying animal matter, and the Cœlopa haunts
seaweed,
occasionally settling on flowers; the Crabro also, as I hear
from Mr. F. Smith, collects small beetles (Halticæ)
for
provisioning its nest. It is equally remarkable, seeing how many kinds
of insects visit this Epipactis, that, although my son
watched for some hours on three occasions hundreds of plants, not a
single humble-bee alighted on a flower, though many were flying about.
In a footnote I have given the results of experiments made by Mr. More,
by cutting off the distal and hinged half of the labellum, in order to
ascertain how far this part is important.1 He has now repeated the
experiment on nine additional flowers: of these, three did not produce
seed-capsules; but this may have been accidental. Of six capsules which
were produced, two contained about as many seeds as the capsules of
unmutilated flowers on the same plant; but four capsules contained much
fewer seeds. The seeds themselves were well-formed. These experiments,
as far as they go, support the view that the distal part of the

labellum plays an important part in leading insects to enter and
leave the flower in a proper manner for fertilization.

Fertilization of Epipactis latifolia (p. 104).—Although
this orchis is not common in the vicinity of Down, by a fortunate
chance several plants sprang up in a gravel walk close to my house, so
that I have been able to observe them during several years, and have
thus discovered how they are fertilized. Although hive-bees and
humble-bees of many kinds were constantly flying over the plants, I
never saw a bee or any Dipterous insect visit the flowers; whilst, on
the other hand, I repeatedly observed each year the common wasp (Vespa
sylvestris) sucking the nectar out of the open cup-shaped
labellum. I thus saw the act of fertilization effected by the
pollen-masses being removed and carried on the foreheads of the wasps
to other flowers. Mr. Oxenden1 also informs me that a large bed of E.
purpurata (which is considered by some botanists a distinct
species, and by others a variety) was frequented by "swarms of wasps."
It is very remarkable that the sweet nectar of this Epipactis should
not be attractive to any kind of bee. If wasps were to become extinct
in any district, so would the Epipactis latifolia.

Dr. H. Müller of Lippstadt has published (Verhandl. d. Nat. Ver.
Jahrg. xxv. III. Folge, v. Bd. pp. 7-36) some very important
observations on the differences in structure and in the manner of
fertilization, as well as on the connecting gradations, between Epipactis
rubiginosa, microphylla and viridiflora. The
latter
species is highly remarkable by the absence of a rostellum, and by
being regularly self-fertilized. This latter circumstance follows from
the incoherent pollen of the lower part of the pollen-masses emitting,
whilst still within the anther-cells, pollen-tubes, which penetrate the
stigma; and this occurred even in the bud state. This
species, however, is probably visited by insects, and occasionally
crossed; for the labellum contains nectar. E. microphylla is
equally remarkable, by being intermediate in structure between E.
latifolia, which is always fertilized by the aid of insects, and E.
viridiflora, which does not necessarily require any such aid. The
whole of this memoir by Dr. H. Müller deserves to be attentively
studied.

Cephalanthera grandiflora (p. 108).—During the year 1862,
the flowers of this orchis appeared to have been visited much less
frequently by insects than during the previous years; for the masses of
pollen were seldom broken down. Although I have repeatedly examined the
flowers, I have never seen a trace of nectar; but some appearances lead
me to suspect that the ridges within the base of the labellum are
attractive to

insects, and are gnawed by them, as in the case of many Vandeæ and
other exotic orchids.

Goodyera repens (p. 114).—Mr. R. B. Thomson informs me
that in the north of Scotland he saw many humble-bees visiting the
flowers and removing the pollen-masses, which were attached to their
proboscides. The bee sent was Bombus pratorum.This
species
grows also in the United States; and Prof. Gray (Amer. Journ. of
Science, vol. xxxiv. 1862, p. 427) confirms my account of its structure
and manner of fertilization, which is likewise applicable to another
and very distinct species, namely, Goodyera pubescens. Prof.
Gray states that the passage into the flower, which is at first very
narrow, becomes, as I suspected, more open during its older state.
Prof. Gray believes, however, that it is the column, and not the
labellum, which changes its position.

Spiranthes autumnalis (p. 123).—As in the case of the Goodyera,
Prof. Gray feels confident that it is the column which moves
from the labellum as the flower grows older, and not, as I had
supposed, the labellum which moves from the column. He adds that this
change of position, which plays so important a part in the
fertilization of the flower, "is so striking that we wonder how we
overlooked it" (Amer. Journ. of Science, vol. xxxiv. p. 427).

On the rostellum of Listera ovata not exploding
spontaneously (p. 149).—I have covered up some additional plants,
and found that the rostellum lost its power of explosion in about four
days, the viscid matter then turning brown within the loculi of the
rostellum. The weather at the time was unusually hot, and this may have
hastened the process. After the four days had elapsed, the pollen had
become very incoherent and some had fallen on the two corners, or even
over the whole surface, of the stigma, which was penetrated by the
pollen-tubes. Hence, if insects should fail to remove the pollinia by
causing the explosion of the rostellum, this orchid certainly seems
capable of occasional self-fertilization. But the scattering of the
incoherent pollen was largely aided by, and perhaps wholly depended on,
the presence of Thrips—insects so minute that they could not
be excluded by any net.

Listera cordata (p. 152).—Prof. Dickie has been so good as
to observe the flowers on living plants. He informs me that, when the
pollen is mature, the crest of the rostellum is directed towards the
labellum, and that, as soon as touched, the viscid matter explodes, the
pollinia becoming attached to the touching object; after the explosion,
the rostellum bends downwards and spreads out, thus protecting the
virgin stigmatic surface; subsequently the rostellum rises and exposes

the stigma; so that everything here goes on as I have described
under Listera ovata. The flowers are frequented by minute
Diptera and Hymenoptera.

On the self-fertilization of Neottia nidus-avis, and
on the rostellum not exploding spontaneously (p. 153).—I covered
up with a net several plants, and after four days found that the
rostellum had not spontaneously exploded, and had already almost lost
this power. The pollen had become incoherent, and in all the flowers
much had fallen on the stigmatic surfaces, which were penetrated by
pollen-tubes. The spreading of the pollen seemed to be in part caused
by the presence of Thrips, many of which minute insects were
crawling about dusted all over with pollen. The covered-up plants
produced plenty of capsules, but these were much smaller and contained
much fewer seeds than the capsules produced by the adjoining uncovered
plants. I may here add that I detected on the crest of the rostellum
some minute rough points, which seemed particularly sensitive
in causing the rostellum to explode.

Dr. H. Müller, of Lippstadt, informs me that he has seen Diptera
sucking the nectar and removing the pollinia of this plant.

On the self-fertilization of certain Epidendreae (p.
166).— Dr. Crüger says (Journ. Linn. Soc. vol. viii. Botany, 1864, p.
131)1 that "we have in Trinidad three plants belonging to the
Epidendreæ (a Schomburgkia, Cattleya, and Epidendron)
which rarely open their flowers, and are invariably impregnated when
they do open them. In these cases it is easily seen that the
pollen-masses have been acted on by the stigmatic fluid, and that the
pollen-tubes descend from the pollen-masses in situ down
into the ovarian canal." Mr. Anderson, a skilful cultivator of orchids
in Scotland, informs me (see also 'Cottage Gardener,' 1863,
p. 206) that with him the flowers of Dendrobium cretaceum never
expand, and yet produce capsules with plenty of seed, which, when
examined by me, was found to be perfectly good. These orchids make a
near approach to those dimorphic plants (as Oxalis, Ononis,
and Viola) which habitually produce open and perfect, as well
as closed and imperfect flowers.

On the slow movement of the pollinia in Oncidium (p.
189).—Mr. Charles Wright,2 in a letter to Prof. Asa Gray, states that he
observed in Cuba a pollinium of an Oncidium attached to a Bombus,
and he concluded at first that I was completely mistaken about the
movement of depression; but after several hours the pollinium moved
into the proper position for fertilizing the flower.

—I may here remark that Delpino (Fecondazione nelle Piante, Firenze,
1867, p. 19) says he has examined flowers of Vanda, Epidendron,
Phaius, Oncidium, and Dendrobium, and confirms
my general statements. The late Prof. Bronn, in his German translation
of this work (1862, p. 221), gives a description of the structure and
manner of fertilization of Stanhopea devoniensis.

Sexes of Acropera not separated (p. 206).—I
have committed a great error about this genus, in supposing that the
sexes were separate. Mr. J. Scott, of the Royal Botanic Garden of
Edinburgh, soon convinced me that it was an hermaphrodite, by sending
me capsules containing good seed, which he had obtained by fertilizing
some flowers with pollen from the same plant. He succeeded in doing
this by cutting open the stigmatic chamber, and inserting the
pollen-masses. My error arose from my ignorance of the remarkable fact
that, as shown by Dr. Hildebrand (Botanische Zeitung, 1863, Oct. 30 et
seq., and Aug. 4, 1865),1 in many orchids the ovules are not
developed until several weeks or even months after the pollen-tubes
have penetrated the stigma. No doubt if I had examined the ovaria of Acropera
some time after the flowers had withered, I should have found
well-developed ovules. In many exotic orchids besides Acropera (namely,
in Gongora, Cirrhæa,
Acineta, Stanhopea, &c.),
the
entrance into the stigmatic chamber is so narrow that the pollen-masses
cannot be inserted without the greatest difficulty. How fertilization
is effected in these cases is not yet known. That insects are the
agents there can be no doubt; for Dr. Crüger saw a bee (Euglossa)
with a pollinium of a Stanhopea attached to its back; and
bees of the same genus continually visit Gongora. Fritz
Müller1 has observed, in the case of Cirrhæa (Bot. Zeitung,
Sept. 1868, p. 630), that if one end of the pollen-mass be inserted
into the narrow entrance of the stigmatic chamber, this part, from
being bathed by the stigmatic fluid, swells, and the whole pollen-mass
is thus gradually drawn into the stigmatic entrance. But, from
observations which I have made on Acropera and Stanhopea
in my own hot-house, I suspect that, with many of these orchids,
the pedicel with the narrow end of the pollinium, and not the broad
end, is ordinarily inserted into the stigmatic chamber. By thus placing
the pollinium, I have occasionally succeeded in fertilizing some of
these orchids, and have obtained seed-capsules.

Structure and fertilization of the Vandeæ &c. of
Brazil (p. 210).—Fritz Müller has sent me many letters containing
an astonishing number of new and curious observations on the structure
and manner of cross-fertilization of various orchids

inhabiting South Brazil. I much regret that I have not here space or
time to give an abstract of his many discoveries, which support the
general conclusions given in my work; but I hope that he will some day
be induced to publish a full account of his observations.

Fertilization of Catasetum (p. 211).—It has been highly
satisfactory to me that my observations and predictive conclusions in
regard to Catasetum have been fully confirmed by the late
Dr. Crüger, the Director of the Botanic Gardens of Trinidad, in letters
to me and in his paper in the 'Journal of the Linnean Society' (vol.
viii. Bot. 1864, p. 127). He sent me specimens of the bees, belonging
to three species of Euglossa, which he saw gnawing the inside
of the labellum. The pollinia, when ejected, become attached to, and
lie flat on, the backs of the bees, on the hairy surface of the thorax.
Dr. Crüger has also proved that I was correct in asserting that the
sexes of Catasetum are separate, for he fertilized female
flowers with pollen from the male plants; and Fritz Müller effected the
same thing with Catasetum mentosum in South Brazil.
Nevertheless, from two accounts which I have received, it appears that Catasetum
tridentatum, though a male plant, occasionally produces
seed-capsules; but every botanist knows that this occasionally occurs
with the males of other diœcious plants. Fritz Müller has given
(Botanische Zeitung, Sept. 1868, p. 630) a most interesting account,
agreeing with mine, of the state of the minute pollinia in the female
plant: the anther never opens, and the pollen-masses are not attached
to the viscid disks, so that they cannot be removed by any natural
means. The pollen-grains, as so generally occurs with rudimentary
organs, are extremely variable in size and shape. Nevertheless the
grains of the rudimentary pollen-masses belonging to the female plant,
when applied (which can never naturally occur) to the stigmatic
surface, emitted their pollen-tubes! This appears to me a very curious
instance of the slow and gradual manner in which structures are
modified; for the female pollen-masses, included within an anther which
never opens, are seen still partially to retain their former powers and
function.

Mormodes luxatum (p. 265).—I have now examined another
species of Mormodes, the rare M. luxatum, and I
find that the chief points of structure, and the action of the
different parts, including the sensitiveness of the filament, are
the same as in M. ignea.The cup of the labellum,
however,
is much larger, and is not pressed down firmly on the filament on the
summit of the column. This cup probably serves to attract insects, and,
as in Catasetum, is gnawed by them. The flowers

are asymmetrical to an extraordinary degree, the right-hand and
left-hand sides differing much in shape.

Cycnoches ventricosum (p. 265).—The plant described in my
work as a second species of Mormodes proves to be Cycnoches
ventricosum. I first received from Mr. Veitch1 some flower-buds,
from which the section (fig. xxx.) was taken; but subsequently he sent
me some perfect flowers. The yellowish-green petals and sepals are
reflexed; the thick labellum is singularly shaped, with its upper
surface convex, like a shallow basin turned upside down. The thin
column is of extraordinary length, and arches like the neck of a swan
over the labellum; so that the whole flower presents a very singular
appearance. In the section of the flower, given in my work, we see the
elastic pedicel of the pollinium bowed, as in Catasetum or Mormodes;
but at the period of growth represented in the figure the pedicel
was still united to the rostellum, the future line of separation being
shown by a layer of hyaline tissue indistinct towards the upper end of
the disk. The disk is of gigantic size, and its lower end is produced
into a great fringed curtain, which hangs in front of the stigmatic
chamber. The viscid matter of the disk sets hard very quickly, and
changes colour. The disk adheres to any object with surprising
strength. The anther is very different in shape from that of Catasetum
or Mormodes,and apparently would retain the
pollen-masses with greater force. A part of the filament of the
anther, lying between two little leaf-like appendages, is sensitive;
and when this part is touched, the pollinium is swung upwards, as in Mormodes,and with sufficient force, if no object stands in the way, to
throw it to the distance of an inch. An insect of large size alights
probably on the labellum, for the sake of gnawing the convex surface,
or perhaps on the extremity of the arched and depending column, and
then, by touching the sensitive point, causes the ejection of the
pollen-masses, which are affixed to its body and thus transported to
another flower or plant.

Fertilization of the Arethuseæ (p. 269).—Epipogium
Gmelini has been the subject of an admirable memoir (Ueber den
Blüthenbau, &c., Göttingen, 1866) by Dr. P. Rohrbach,2 who has shown
how the flowers are fertilized by Bombus lucorum.With
respect to another genus belonging to this same tribe, namely Pogonia,
Dr. Scudder3 of the United States has described (Proc. Boston Nat. Hist.
Soc. vol. ix. 1863, p. 182) the manner in which it is fertilized by the
aid of insects.

Cypripedium (p. 274).—Prof. Asa Gray, after examining
several American species of Cypripedium, wrote to me (see
also Amer. Journ. of Science, vol. xxxiv. 1862, p. 427) that

he was convinced that I was in error, and that the flowers are
fertilized by small insects entering the labellum through the large
opening on the upper surface, and crawling out by one of the two small
orifices close to either anther and the stigma. Accordingly I caught a
very small bee which seemed of about the right size, namely the Andrena
parvula (and this by a strange chance proved, as we shall
presently see, to be the right genus), and placed it in the labellum
through the upper large opening. The bee vainly endeavoured to crawl
out again the same way, but always fell backwards, owing to the margins
being inflected. The labellum thus acts like one of those conical traps
with the edges turned inwards, which are sold to catch beetles and
cockroaches in the London kitchens. Ultimately the little bee forced
its way out through one of the small orifices close to one of the
anthers, and was found when caught to be smeared with the glutinous
pollen. I then again put the same bee into the labellum; and again it
crawled out through one of the small orifices. I repeated the operation
five times, always with the same result. I then cut away the labellum,
so as to examine the stigma, and found it well smeared over with
pollen. Delpino (Fecondazione &c. 1867, p. 20) with much sagacity
foresaw that some insect would be discovered to act in the manner just
described; for he argued that if an insect were to insert its
proboscis, as I had supposed, from the outside through one of the small
orifices close to one of the anthers, the stigma would be fertilized by
the plant's own pollen; and in this he did not believe, from having
confidence in what I have often insisted on—namely, that all the
contrivances for fertilization are arranged so that the stigma shall
receive pollen from a distinct flower or plant. But these speculations
are now all superfluous; for, owing to the admirable observations of
Dr. H. Müller, of Lippstadt (Verh. d. Nat. Ver. Jahrg. xxv. III. Folge,
v. Bd. p. 1), we actually know that Cypripedium calceolus in
a state of nature is fertilized by two species of Andrena, in
the manner above supposed.

On the relation between the more or less viscid condition of the
pollen and stigma in Cypripedium (p. 276).—The relation between
the state of the pollen and stigma, which I have pointed out in my
work, is strongly confirmed by Prof. Gray's statement (Amer. Journ. of
Science, vol. xxxiv. 1862, p. 428), namely, that in C. acaule the
pollen is much more granular or less viscid than in other American
species of the genus, and in this species alone the stigma is slightly
concave and viscid! Dr. Gray adds that in most of the species the broad
stigma presents another remarkable peculiarity, "in being closely

beset with minute, rigid, sharp-pointed papillæ, all directed
forwards, which are excellently adapted to brush off the pollen from an
insect's head or back."

The use of the copious fluid contained within the labellum of Coryanthes
(p. 278).—The Coryanthes macrantha is perhaps the
most wonderful of all known orchids, even more wonderful in structure
and function than Catasetum.Its manner of
fertilization has
been described by Dr. Crüger in the 'Journal of the Linnean Society'
(vol. viii. Bot. 1864, p. 130), and in letters to me. He sent me bees,
belonging to the genus Euglossa,which he saw at
work. The
fluid in the bucket formed by the basal part of the labellum is not
nectar and does not attract insects, but serves, by wetting their
wings, to prevent them from crawling out except through the small
passages close to the anther and stigma. Thus the secretion of fluid in
this orchis serves exactly the same end as the inflected margins of the
labellum in Cypripedium.

On the evidence that Insects visit many exotic Orchids in
order to gnaw parts of the labellum, and not for the sake of nectar (p.
284).—It has been highly satisfactory to me that this hypothesis has
been fully confirmed. In the West Indies, Dr. Crüger witnessed
humble-bees of the genus Euglossa gnawing the labellum of Catasetum,
Coryanthes, Gongora, and Stanhopea; and Fritz
Müller
has repeatedly found, in South Brazil, the prominences on the labellum
of Oncidium gnawed. We are thus enabled to understand the
meaning of the various extraordinary crests and projections on the
labellum of various exotic orchids; for they invariably stand in such a
position that insects, whilst gnawing them, will be almost sure to
touch the viscid disks of the pollinia, and thus remove them.

Bonatea speciosa (p. 305).—The manner of fertilization of
this extraordinary orchis has now been fully described by Mr. R. Trimen
in the 'Journal of the Linnean Society' (vol. ix. Bot. 1865, p. 156). A
projection rising from the base of the labellum is one of its most
remarkable peculiarities, as an insect is thus compelled to insert its
proboscis on one side, and thus to touch one of the two widely
separated and projecting viscid disks. Mr. J. P. Mansel Weale1 has also
published (ibid. vol. x. 1869, p. 470) analogous observations
on a second species, viz. Bonatea Darwinii. Mr.
Weale caught
a skipper-butterfly (Pyrgus elmo) quite embarrassed by the
number of pollinia belonging to this orchis which adhered to its
sternum. I do not know of any other case in which the pollinia adhere
to the sternum of a Lepidopterous insect.

On the nature of the contraction which causes the pollinia,
after their removal from the anther, to change their
position

(p. 338).—In Orchis hircina, I clearly saw, under the
microscope, the whole front of the viscid disk become depressed as the
two pollinia together underwent the movement of depression.

Number of seeds (p. 344).—The number of seeds produced by Orchis
maculata, as given in my work, is small in comparison with that
produced by some foreign species. Ihave shown (Variation of
Animals and Plants under Domestication, vol. ii. 1868, p. 379), on the
authority of Mr. Scott, that a single capsule of Acropera contained
371,250 seeds; and the species produces so many flowers and racemes,
that a single plant probably sometimes produces as many as 74 millions
of seeds in the course of a single year. Fritz Müller carefully
estimated, by weighing, the number of seeds in a single capsule of a Maxillaria
in South Brazil, and found the number 1,756,440. The same plant
sometimes produces half-a-dozen capsules.

Number of pollen-grains (p. 355).—I have endeavoured to
estimate the number of pollen-grains produced by a single flower of Orchis
mascula.There are two pollen-masses; in one of these I
counted
153 packets of pollen; each packet contains, as far as I could count,
by carefully breaking it up under the microscope, nearly 100 compound
grains; and each compound grain is formed of four grains. By
multiplying these figures together, the product for a single flower is
about 120,000 pollen-grains. Now we have seen that in the allied O.
maculata a single capsule produced about 6,200 seeds; so that
there are nearly twenty pollen-grains for each ovule or seed. As a
single flower of a Maxillaria produced 1,756,000 seeds, it
would produce, according to the above ratio, nearly 34 million
pollen-grains, each of which, no doubt, includes the elements for the
reproduction of every single character in the mature plant!

Enumeration of the Orchideæ which, as at present
known, habitually fertilize themselves (p. 358).—We have now seen
that self-fertilization habitually occurs, in a more or less perfect
manner, in one of the species of Ophrys, of Neotinea,
Gymnadenia, Platanthera, Epipactis,
Cephalanthera, Neottia, and
in those Epidendreæ and in Dendrobium
which often
produce flowers that never expand. No doubt other cases will hereafter
be discovered. Self-fertilization seems to be more perfectly secured in
Ophrys apifera and in Neotinea intacta than in the
other species. But it deserves especial notice that in all these
orchids structures are still present, not in a rudimentary condition,
which are manifestly adapted for the transport by insects of the
pollen-masses from one flower to another. As I have elsewhere remarked,
some plants, both indigenous

and naturalized, rarely or never bear flowers, or, if they do bear
flowers, these never produce seed. But no one doubts that it is a
general law of nature that phanerogamic plants should produce flowers,
and that these flowers should produce seed. When they fail to do this,
we believe that such plants would perform their proper functions under
different conditions, or that they formerly did so and will do so
again. On analogical grounds I believe that the few orchids which do
not now intercross, either did formerly intercross (the means for
effecting this being still retained) or that they will do so at some
future period under different conditions, unless, indeed, they become
extinct from the evil effects of long-continued close interbreeding.